`subprocess.stdout` can be undefined instead of null

[unarist]

Affected URL(s)

https://nodejs.org/docs/latest-v16.x/api/child_process.html#subprocessstdout

Description of the problem

The subprocess.stdout property can be null if the child process could not be successfully spawned.

However, it can be undefined in some cases:

$ docker run --rm -it node:16 /bin/bash
root@c12d92a299db:/# ulimit -n 17
root@c12d92a299db:/# node -e 'p=require("child_process").spawn("uname");p.stdout.on("data",console.log)'
[eval]:1
p=require("child_process").spawn("uname");p.stdout.on("data",console.log)
                                                   ^

TypeError: Cannot read properties of undefined (reading 'on')
    at [eval]:1:52
    at Script.runInThisContext (node:vm:129:12)
    at Object.runInThisContext (node:vm:305:38)
    at node:internal/process/execution:76:19
    at [eval]-wrapper:6:22
    at evalScript (node:internal/process/execution:75:60)
    at node:internal/main/eval_string:27:3
root@c12d92a299db:/# node -e 'console.log(require("child_process").spawn("uname"))'
<ref *1> ChildProcess {
  _events: [Object: null prototype] {},
  _eventsCount: 0,
  _maxListeners: undefined,
  _closesNeeded: 1,
  _closesGot: 0,
  connected: false,
  signalCode: null,
  exitCode: null,
  killed: false,
  spawnfile: 'uname',
  _handle: Process {
    onexit: [Function (anonymous)],
    [Symbol(owner_symbol)]: [Circular *1]
  },
  spawnargs: [ 'uname' ],
  [Symbol(kCapture)]: false
}
node:events:505
      throw er; // Unhandled 'error' event
      ^

Error: spawn uname EMFILE
    at Process.ChildProcess._handle.onexit (node:internal/child_process:283:19)
    at onErrorNT (node:internal/child_process:478:16)
    at processTicksAndRejections (node:internal/process/task_queues:83:21)
Emitted 'error' event on ChildProcess instance at:
    at Process.ChildProcess._handle.onexit (node:internal/child_process:289:12)
    at onErrorNT (node:internal/child_process:478:16)
    at processTicksAndRejections (node:internal/process/task_queues:83:21) {
  errno: -24,
  code: 'EMFILE',
  syscall: 'spawn uname',
  path: 'uname',
  spawnargs: []
}

I think this should be documented.

[cola119]

This is because ChildProcess.prototype.spawn returns the error before ChildProcess.stdout is initialized to null at L457.

node/lib/internal/child_process.js

Lines 388 to 401 in 3473d1b

	const err = this._handle.spawn(options);
	// Run-time errors should emit an error, not throw an exception.
	if (err === UV_EACCES ||
	err === UV_EAGAIN ||
	err === UV_EMFILE ||
	err === UV_ENFILE ||
	err === UV_ENOENT) {
	process.nextTick(onErrorNT, this, err);
	// There is no point in continuing when we've hit EMFILE or ENFILE
	// because we won't be able to set up the stdio file descriptors.
	if (err === UV_EMFILE || err === UV_ENFILE)
	return err;

Which is better to document it can be null and undefined, or to fix to initialize ChildProcess.stdout to null as an initial value at the top of the function?

[unarist]

I think it would be better if stdout has same value when the spawn can't create pipes, for consistency and maybe ease to handle.

I'm okay for documenting the behavior, though.